There are several standards for mobile telephone communications. Each of these standards requires precise output power control over a large dynamic range. In order to provide output power control, mobile telephone architectures typically employ one or more power amplifiers that amplify a signal prior to transmission. However, the output of the power amplifier fluctuates due to variations in the characteristics of the power amplifier caused by changes in temperature. Thus, in order to provide precise output power control over a large dynamic range, some form of temperature compensation is desirable to compensate for variations in the output of the power amplifier due to variations in the characteristics of the power amplifier caused by changes in temperature.
The need for temperature compensation can be more clearly described with respect to FIG. 1. FIG. 1 illustrates a basic system 10 for controlling the output power of a mobile telephone having no temperature compensation. The system 10 includes a power amplifier 12 receiving a supply voltage (VCC) from a voltage regulator 14. The voltage regulator 14 generates the supply voltage (VCC) from a source voltage (VSOURCE), such as a battery, based on a ramp voltage (VRAMP). The system 10 may be part of a polar modulation system, wherein the ramp voltage (VRAMP) is an amplitude signal corresponding to an amplitude component of data to be transmitted and the input signal (INPUT) is a carrier signal modulated by a phase component of the data to be transmitted. Based on the ramp voltage (VRAMP), the power amplifier 12 modulates the input signal (INPUT) by the amplitude signal. However, since the system 10 does not provide temperature compensation for the power amplifier 12, changes in temperature cause the output of the power amplifier 12 to fluctuate, thereby introducing error into the output signal (OUTPUT) and degrading the performance of the polar modulation system.
The system 10 may also be used to control the output power of a transmitter. In this case, the ramp voltage (VRAMP) is a control voltage that controls the output power of the power amplifier, thereby controlling the output power of the transmitter. However, since the system 10 does not provide temperature compensation for the power amplifier 12, changes in temperature result in fluctuations in the output power of the power amplifier.
Thus, there remains a need for a system that compensates for variations in the output power of a power amplifier due to temperature variations.